The high pressure die casting process accounts for about 70% of the annual tonnage of all aluminum castings in the United States. In the high pressure die casting process, a shot sleeve chamber connected to the die mold cavity receives molten metal poured slowly by a gravity process through a relatively small hole located distally from the die cavity. The molten metal stream impacts an inside diameter of the shot sleeve opposite the small entry hole and subsequently fills the shot sleeve chamber by flowing toward the die cavity. Once the slot sleeve chamber is filled, high pressure is applied to quickly force the molten metal into the die cavity. In the die cavity, the molten metal fills the cavity in 30 to 100 milliseconds and then the molten metal is pressed against the die surface where it has the opportunity to alloy or solder to the steel die surface of the tooling as it cools and solidifies.
Traditionally, the alloys used in high pressure die casting contain about 1% iron to provide die soldering resistance. For example, the aluminum alloy ingot and casting iron concentration limits for 360, 364, 381, 383, 384, and 390 are as listed below and reflect the fact that during the die casting process, the ingot can incorporate iron from the die. The allowance for iron incorporation during the die casting process allows for the subtraction of the upper limit or maximum of the iron range specified for the ingot from the maximum iron (Fe) specification for the casting. Thus, the specification for the casting is provided only as a “max Fe” specification because the counterpart specification for the ingot provides the “min Fe” specification. Accordingly, the ingot specification and the casting specification together provide a minimum and maximum Fe range for a particular alloy composition. Specifically:                Ingot 360.2 has an iron range of 0.7-1.1% Fe and casting 360.0 has an iron max of 2.0% Fe;        Ingot 364.2 has an iron range of 0.7-1.1% Fe and casting 364.0 has an iron max of 1.5% Fe;        Ingot 380.2 has an iron range of 0.7-1.1% Fe and casting 380.0 has an iron max of 2.0% Fe;        Ingot 381.2 has an iron range of 0.7-1.0% Fe and casting 381.0 has an iron max of 1.3% Fe;        Ingot 383.2 has an iron range of 0.6-1.0% Fe and casting 383.0 has an iron max of 1.3% Fe;        Ingot 384.2 has an iron range of 0.6-1.0% Fe and casting 384.0 has an iron max of 1.3% Fe;        Ingot 390.2 has an iron range of 0.6-1.0% Fe and casting 390.0 has an iron max of 1.3% Fe.        
This high level of iron of about 1% in the alloy compositions shown above severely degrades the mechanical properties of the resultant castings, particularly the ductility of the castings. Use of such alloys in high pressure die castings is therefore limited to low mechanical property applications. This problem is not simply cured by lowering the iron level to sand casting alloy and/or permanent mold casting alloy levels because a low iron level in such alloys fails to prevent the molten aluminum alloy from fully or partially soldering with a conventional die casting metal mold (generally constructed of H-13 tool steel). Thus, the die cast part may not be able to be removed or ejected from the die cavity without soldering damage to the part and/or the die. With this knowledge, iron concentrations of 0.8% or more are traditionally used for high pressure die casting operations to reduce the tendency of the casting to solder to die casting tooling.
This reduced die soldering tendency due to high iron content results because the Al—Fe—Si ternary eutectic composition occurs at about 0.8% Fe. Theoretically, when the iron constituency in the molten alloy is at or above 0.8% Fe, the molten alloy has little tendency to dissolve the relatively unprotected tool steel while the molten alloy and die or shot sleeve are in intimate contact because the molten alloy is already supersaturated with iron. This “bulk” non-alloying effect is what observers attribute the die soldering resistance to in traditional high iron containing die casting alloys.
In response to the ductility performance concerns with the high iron containing die casting alloys noted above, several newly developed die casting alloys that are low in iron content have been developed for high performance applications in the high pressure die casting process. Two of these alloys are SILAFONT-36™ and AURAL-2™ which have the Aluminum designation 365.0 [9.5-11.5% Si, 0.15% max Fe, 0.03% max Cu, 0.50-0.8% Mn, 0.10-0.50% Mg, 0.07% max Zn, 0.04-0.15% Ti, other-each 0.03% and other-total 0.10%] and A365.0 respectively. SILAFONT-36™ and AURAL-2™ both rely on manganese for die soldering resistance. The other three high-performance, low iron die casting alloys rely on strontium for their die soldering resistance. The designations for these die casting alloys are:                367.0: 8.5-9.5% Si, 0.25% max Fe, 0.05-0.07% Sr, 0.25% max Cu, 0.25-0.35% Mn, 0.30-0.50% Mg, 0.10% max Zn, 0.20% max Ti, other-each 0.05%, and other-total 0.15%        368.0: 8.5-9.5% Si, 0.25% max Fe, 0.05-0.07% Sr, 0.25% max Cu, 0.25-0.35% Mn, 0.10-0.30% Mg, 0.10% max Zn, 0.20% max Ti, other-each 0.05%, and other-total 0.15%        362.0: 10.5-11.5% Si, 0.40% max Fe, 0.05-0.07% Sr, 0.20% max Cu, 0.25-0.35% Mn, 0.50-0.7% Mg, 0.10% max Ni, 0.10% max Zn, 0.20% max Ti, 0.10% max Sn, other-each 0.05%, and other-total 0.15%        
There are significant compositional differences between the traditional high iron die casting alloys and the newly developed low iron die casting alloys that rely on strontium for their die soldering resistance. The traditional high iron containing alloys rely on high bulk alloying effect levels of iron to prevent die soldering. Similarly, SILAFONT-36™ and AURAL-2™ while containing low levels on iron, rely on high bulk alloying effect levels of manganese to prevent die soldering. In contrast, the strontium containing low iron alloys rely on surface phenomena effects created with the strontium addition to prevent die soldering. Moreover, this is accomplished with only one tenth the concentration of strontium as compared to the iron concentration in the traditional high iron alloys.
Traditional high-iron die casting alloys have low ductility because they contain the iron needle-like phase Al5FeSi phase that acts as a severe stress riser in the microstructure. SILAFONT-36™ and AURAL-2™ have higher mechanical properties than these traditional high iron die casting alloys, but the bulk alloying effects create intermetallic manganese phases in the microstructure that are also stress risers, although to a lesser degree than the needle-like iron phase stress risers. In sharp contrast, the low iron, strontium containing die casting alloys do not contain intermetallic compounds or either strontium or manganese in the microstructure and therefore have the highest strain rate impact properties of the die casting alloys discussed above.
The differences between the alloys continue when the effects of shot sleeve washout and/or erosion are examined and analyzed. Prior to being injected into the die cavity, the metal is at its highest temperature when it is poured into the shot chamber through the hole in the shot sleeve farthest from the die cavity impacting an internal surface of the shot sleeve at an impingement site. With the new, low iron die casting alloys the molten metal tends to erode this impingement site where the molten metal hits the inside diameter of the shot sleeve opposite the small entry hole. Both the manganese containing die casting alloys (SILAFONT-36™ and AURAL-2™) and the strontium die casting alloys (AA designations 362, 367, and 368) exhibit good die soldering resistance in the die cavity but very poor erosion resistance at the impingement site in the shot sleeve. This creates a very serious problem due to excessive replacement costs of the die tooling, particularly the shot sleeves.
Low iron, strontium containing die casting alloys exist in a relatively static situation in the die cavity of the tooling where a thin strontium oxide or strontium aluminate film exists between the molten metal and the die, to account for die soldering prevention. Alternatively, the strontium oxide, SrO, may react with the alumina, Al2O3, and form strontium aluminate, SrAl2O4, as the barrier between the die cavity surface and the molten aluminum alloy. However, in the high pressure die casting process, when the molten metal is injected into the shot sleeve, the oxide layer on the molten metal is continuously disturbed or displaced upon impact with the inside diameter of the shot sleeve opposite the entry hole.
When molten aluminum contacts the surface of the shot sleeve the shot sleeve is heated by the molten aluminum. The turbulent flow of the molten metal breaks down any naturally occurring oxide coating that might have been on either the molten aluminum (e.g. strontium oxide or strontium aluminate) or the iron based alloy of the shot sleeve. As a result, iron dissolves into the molten aluminum and aluminum diffuses into the iron based alloy shot sleeve.
Convection currents in the turbulent molten aluminum cause any iron dissolved in the molten aluminum to be carried into the bulk liquid by fluid flow along the shot sleeve and into the die cavity, and eventually to be entrained in the casting. Thus, a quasi-equilibrium condition results wherein the iron concentration on the aluminum side of the melt/sleeve interface is low. Further, the rate that iron dissolves into the melt decreases as intermetallic compounds form and grow on the shot sleeve surface. This diffusional or kinetic mechanism indicates that the transport of aluminum into the shot sleeve material is of paramount importance for washout, and the transport of iron dissolving in aluminum and into the casting is relatively unimportant.
Thus, turbulence in the molten metal stream effectively negates the beneficial die soldering resistance provided by the thin strontium oxide or strontium aluminate film on the molten metal of 362, 367 and 368 in static situations present after filling in the die cavity. The erosion by the low iron containing die casting alloy is located at the inside diameter location of the shot sleeve below the pouring hole where the turbulent flow interfaces with the shot sleeve. Similarly, the low iron, manganese containing SILAFONT-36™ and AURAL-2™ both suffer the same severe alloying and or erosion of the shot sleeve due in part to the hotter, turbulent molten metal that enters the shot sleeve through the pouring hole and impacts the shot sleeve ID surface below the pouring hole and severely disrupts any protective oxide film on the molten metal more than that which enters and is pressed into the die cavity.
The result is that when the new low iron die casting alloys are used, the shot sleeve has only about a 10 to 20% of the life as compared to shot sleeves used with traditional high iron die casting alloys containing high levels of iron. While water cooling the shot sleeve produces marginal improvements in the life of shot sleeves used with low iron die casting alloys, this approach is not a solution to the broader erosion problem. Further, shot sleeves made from variants of H-13 steel, such as DIEVAR™ and QRO-90™, exhibit no significant increase in shot sleeve life when the die casting alloy is low in iron.